Put a large pot of salted water over high heat; this will be your pasta pot.

In a large skillet, warm a splash of olive oil and the butter over medium heat. Add the red onion, celery, carrot, and Italian parsley, and cook, stirring occasionally, until the vegetables are translucent. Add the zucchini blossoms, a pinch or two of salt, and the saffron, and stir gently to mix. Add about ¾ cup of broth, and stir to combine. Raise the heat to medium-high and add the rest of the broth a splash or two at a time, taking about 5-8 minutes to add it all. Stir frequently. Allow the sauce to simmer until most of the liquid has evaporated and only a thin film of thickened broth remains in the pan. Remove from the heat.

In a small bowl, whisk the egg yolk slightly with a fork.

Meanwhile, cook the pasta until tender but al dente. When the pasta is almost ready, place the zucchini blossom sauce back over medium heat. Use a small measuring cup to scoop up about 3 Tbs of pasta water and, whisking constantly with a fork, gradually add the hot water to the egg yolk: together, they should make a loose, pale yellow liquid. Pour this mixture into the sauce in the skillet, stirring well. Using tongs or a spider, scoop the finished pasta from its pot into the skillet, and toss with the sauce over medium heat for about 30 seconds.